This invention relates to torque motors, and, in particular to a torque motor having a rotor and a stator configured for minimizing off-axis forces on the rotor.
Torque motors are widely used for a variety of applications. One application is the use of a torque motor, in combination with a shaft position transducer, to drive optical elements in order to guide light beams. The optical element in this device can be a lens, mirror, waveplate, or the like. This type of device is often called an optical scanning galvanometer or optical scanner. There are three basic types of torque motors that are known and could be used in such an application. These include a moving coil design, a moving iron pole construction, and a moving magnet type. With the introduction of high energy or rare earth permanent magnets, the moving magnet type has become the preferred type of torque motor for optical scanners. The moving magnet design has a stationary coil, armature, and a rotating magnet that defines the field. This type of construction has a high torque constant, low inertia, low electrical inductance, and effective thermal cooling of the stationary coil.
A typical prior art moving magnet torque motor has a cylindrical sleeve that encompasses a rotating magnet. Coils made up of a multiplicity of turns of wire are arranged along the inside surface of the cylindrical sleeve. The coils reverse direction between the sleeve and the magnet. The length of the sleeve is longer than the length of the magnet. Therefore, the turn around zones are disposed circumferentially around the inner edge of the sleeve and lie within the length of the magnet. When a supplied current flows through the coils, magnetic flux lines emanate from the coils to act on the magnet. These magnetic flux lines are not directly offsetting given the position of the turn around zones relative to the magnet. This results in off-axis forces applied to the magnet and a corresponding loss of speed and accuracy for the motor.
Accordingly, there is a need in the art for a reliable motor in which off-axis forces acting on a moving magnet resulting from magnetic flux lines emanating from energized coils are minimized.
The present invention relates to a torque motor configuration that minimizes off-axis magnetic forces generated by adjacent energized coils acting on a magnet. In particular, a motor consistent with the invention may include: a stator comprising a sleeve with conductive coils disposed thereon with the stator further defining a rotor opening; and a rotor disposed in the rotor opening comprising a magnet disposed on a shaft, the sleeve being shorter than the magnet and the coils including turn around zones beyond respective ends of the sleeve. Upon energization of the coils, a magnetic field is established that interacts with the magnet""s magnetic field to impart torque on the rotor.
The magnet may be a permanent magnet or an electromagnet. There may also be a magnetically permeable outer housing enclosing the sleeve where the housing has annular slots to accept and hold the coil turn around zones.
In an exemplary application, an exemplary torque motor may be part of optical scanning system. The optical scanning system may include an optical element to direct light from a light source, and a torque motor as described earlier. The optical element may be a mirror, waveplate, or lens.